A visitor to Little Round Top views the Devil's Den during ongoing activities commemorating the 150th anniversary of the Battle of Gettysburg on Monday in Gettysburg, Pa.

Written by

Allen C. Guelzo
For USA TODAY

There are a few moments in history when the normal, unhurried pace of events undergoes a sudden, violent compression, when single decisions become the starts of avalanches and people find the compass points of their lives spinning wildly askew. We have experienced a few of these moments in living memory - Pearl Harbor, the Kennedy assassination - but the largest one in U.S. history was the Battle of Gettysburg, whose 150th anniversary looms before us this week.

Gettysburg was the midpoint of our four-year Civil War. But it also became the war's hinge-point, and it's not difficult to see why.